Eddy covariance measurements of energy and CO 2 fluxes over a boreal lake in southern Finland

Transcription

1 Lake Workshop, September 2012, Helsinki Eddy covariance measurements of energy and CO 2 fluxes over a boreal lake in southern Finland Ivan Mammarella 1, Ü. Rannik 1, S. Haapanala 1, A. Nordbo 1, J. Pumpanen 2, A. Ojala 3, T. Vesala 1 1 Department of Physics, University of Helsinki 2 Department of Forest Ecology, University of Helsinki 3 Department of Environmental Sciences, University of Helsinki

2 Background and Motivation Worldwide, lakes cover 3% of the Earth s continental surface; in Finland >>> 10 %. About 95% of lakes are less than 1 Km 2. The important role of inland fresh waters (lakes, rivers, ponds) in processing large amounts of organic carbon (Battin et al., 2009, Nature Geoscience). Lakes have different albedo, lower surface roughness and greater effective heat capacity than surrounding land areas. These properties affect regional climate and energy budgets (Dutra et al., 2010; Jeffries et al., 1999). Only few studies have been published on EC flux measurements on fresh water ecosystems. Long term and continuous flux measurements are needed in order to improve our understanding on: - short and long term variation of thermal structure and energy balance of such ecosystems. - gas exchange processes

3 UHEL EC flux measurement sites Peat soil Siikaneva I boreal fen Mineral soil Urban environment Lakes Siikaneva II boreal bog Hyytiälä Scots pine (SMEAR II) Lake Kuivajärvi ( Lake Valkea-Kotinen Kumpula Urban (SMEAR III) Hotel Torni Urban

4 Materials and methods Max depth : 13.2 m. Length = 2.6 km. Max width = 400 m. Source: Maanmittauslaitos

5 Materials and methods Ancillary measurements: Water T Profile: 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 8, 10, 12. Water CO2 Profile: 0.2, 0.5, 1, 1.5 Water PAR. Radiation components. Tair, RH Measurement Periods: From J.Heskainen MSc thesis June 2009-Feb 2010 June 2010 Jan 2011 May 2011 Nov 2011

6 EC measurements: 1) June 2009 / Feb 2010 and Jun 2010/Dec Metek USA-1 (momentum and sensible heat fluxes) Licor 7000 (CO2 and H2O fluxes) 2) Jun 2011 / Nov 2011 Metek USA-1 (momentum and sensible heat fluxes) Licor 7200 (CO2 and H2O fluxes) EC data post-processing by EddyUH UHsoftware.php.

7 EC system performance

8 EC fluxes: systematic uncertainty Spectral correction is up to 30% for H2O and 20% for CO2.

9 Environmental conditions

10 Environmental conditions Wind direction at Kuivajärvi is dominantly along lake.

11 Environmental conditions Themocline depth

12 Next we consider only open water periods (e.g. June October)

13 Lake vs Scots Pine forest Sept 2011 Red = Kuivajärvi, Green = SMEAR II

14 Bulk transfer relationships

15 Bulk transfer relationships

16 Bulk transfer relationships WHY? -Buoyancy play relevant role. - Bias in CO2 concentration measurements. - EC flux random uncertainty

17 Flux random uncertainty SH CO2 LE RFRE 17% 35% 20% RE 2.5 W m mumol RFRE m= -2 RE/Flux s -1 4 W m -2 Flux random uncertainty according to Finkelstein & Sims (2001), JGR

18 Water turbulent mixing and gas exchange F k( C C ) aq k is the gas transfer velocity, which depends on molecular diffusivity of the gas and the MBL variation. air

19 Convective mixing : some equations! We used theoretical framework similar to atmospheric mixed layer, but we apply it to water side (Imberger, 1985). Convective velocity scale is 1 3 Z is the mixed layer depth and beta is the buoyancy flux: w* Z( ) gh C p eff w = 1.6e-5+9.6e-6*Tw is the thermal expansion of water Cp = 4192 J/Kg/K is the specific heat capacity w is water density

20 The effective heat flux Heff is defined as 2 Heff H net q(0) q( Z) q( z) dz Z where H LW H LE is the net surface heat flux and net net 0 z q is the short wave radiation q( z) q(0) exp( z) is the light attenuation coeff, taken equal to 3.2 m-1 Beer s law is applied to PAR measurements to quantify the penetration of radiation in the water column. Sign convection for Heff: it is negative when the lake is cooling, and positive when the lake is heating. Note that during night-time Heff = Hnet.

21 Diurnal variation of water stratification and mixed layer depth during summer Thin mixed layer Deeper mixed layer ML depth was estimated from temperature profile as the depth where the temperature deviates more than 0.2 Celsius degree from the surface temperature.

22 Example (15-25 Aug 2010) Strong diurnal cycle Front breaks the diurnal cycle

23 Example (15-25 Aug 2010) W* > u*

24 Gas transfer velocity versus wind speed

25 Near-future plans... EC measurements started again in June 2012 (data have been already post-processed). On-going EC CH4 flux measurement campaign (Aug 2012 Nov 2012).

26 Turbulence measurement in the water by ADV/microstructure profiler (September 2012 and during next year). Compare EC flux with chamber flux. Systematic analysis of buoyancy driven mixing events. Analysis of energy balance and exchange processes during winter periods. Data are available for model validation and comparison studies.

27 THANKS!!!